This invention relates to means for concentrating solar radiation by use of a plurality of collectively controlled mirrors.
For many desired applications of solar energy, it is highly desirable, and in some cases it is essential, that the beam component of solar radiation be concentrated to flux levels many times greater than the ambient value. The need for an economical means of providing highly concentrated solar radiation is present both for small scale and large scale applications. For example, the use of solar energy for air conditioning, either by conversion of thermal energy to mechanical energy which is used to power the air conditioner, or by absorption cooling, requires the higher temperatures available from concentrating type collectors to achieve good efficiency. The present extremely high cost of photoelectric cells for direct conversion of solar to electrical energy can be counteracted by the use of sufficiently high flux concentration coupled with rapid heat removal from the photo cell. Effective thermal energy storage can best be accomplished at the higher temperatures available from concentrating collectors. More sophisticated storage methods, such as the reversible thermochemical conversion of alkaline earth hydroxides to oxides, as described in U.S. Pat. Nos. 3,973,552 and 3,955,554, are dependent on the availability of collectors providing high flux concentration. The feasibility of large scale thermal conversion solar power plants likewise depends on the development of economical collectors which provide high flux concentration.
A large number of types of concentrating collectors are known. One of the most comprehensive inventories of the known types is the book Applied Solar Energy: An Introduction by A. B. Meinel and M. P. Meinel, (Addison-Wesley, New York, 1976). Despite the large number of available types of concentrating collectors, it is not yet clear that any of these can be manufactured at a cost which will allow solar energy to make a major contribution for applications requiring substantial flux concentration.
One of the most obvious types of concentrating collector is simply a combination of a parabolic mirror with an appropriate radiation absorbing receiver placed at the focus. A tracking mechanism such as that described in U.S. Pat. No. 3,305,686, which is hereby incorporated for reference, allows the collector to follow the sun. A primary difficulty with collectors of this type is that the tracking mechanism is too complex and expensive to be practical for small collectors; while the structural requirements for maintaining the parabolic mirror shape cause the cost to rise much faster, as the collector size is increased, than does the area of the collector, so large collectors are also impractical.
One of the concentrating collector concepts which is currently receiving a good deal of attention is the possible large scale application in which a radiation receiver is placed on a high tower and furnished with concentrated solar radiation by a large number of independently guided mirrors mounted near ground level. Among the advantages of this system is the fact that a great deal of solar energy is collected for use at the single central power plant without the requirement for transporting fluid at very high temperatures over large distances as is required in some of the alternative large scale power plant proposals. The tracking accuracy requirements for each mirror are even more formidable than the accuracy requirements for an independent parabolic collector. However, the scale of the collector is to be so large that the individual mirrors may be generally plane surfaced mirrors rather than parabolic (although the use of multiple faceted mirrors may be desirable), thus allowing more economical mirrors to be used than in alternative collector concepts.
Because of the current interest in the type of large scale collector concept described above, a number of articles have recently appeared dealing with the technical details of specific embodiments of this type of collector. For example, in the first two issues of Volume 19 of the journal Solar Energy may be found articles dealing with: optimum mirror deployment, mirror aiming strategy, optimum receiver design, and the expected performance of metallized plastic film mirrors when used with central tower solar collectors. These detailed topics are all of interest relative to the invention to be described. However, of more specific interest is the article "Solar Thermal Power System Based On Optical Transmission" by L. L. Vant-Hull and A. F. Hildebrand, Solar Energy 18, 31-39(1976). This article presents a summary of a rather detailed technical and economic evaluation of the major components making up a full size solar power plant. A notable conclusion of this study is that despite the apparent technical difficulty of locating a major part of a power plant at the top of a tower which, according to this study, would be among the tallest man-made structures in the world, the cost of the tower and receiver are found to be only a "minor cost component". The dominating cost of the collector is in the field of heliostats, and if there are to be any cost reductions sufficient to substantially increase the practicality of large scale solar power, the cost reduction will have to come in the overall cost per unit area of mirror in the heliostat field.
In the study cited above, a number of types of mounting systems were considered. An elevation-azimuth mount was found to be most desirable. The guidance system selected was a combination of course control by a central computer and fine control by an individual optical sensor associated with each mirror. Local mini-computers are required to coordinate the guidance systems. Tracking accuracy is to be within one milliradian. Various mirror systems were also considered, with the final choice being a steel backed 20 ft. glass mirror. The overall heliostat costs broke down roughly as one fourth for electronics and computer, one fourth for actuators, and the remaining one half for mirror structure and pedestal. The cost per unit area of the final heliostat package was calculated as roughly 20 times as great as the cost per unit area of an unmounted glass mirror of the quality used in the heliostat.
It will be realized that the heliostat component costs are an interrelated package. The amount of structural material incorporated into the mirror backing and the pedestal may be expected to scale roughly as the cube of the linear dimensions of the mirror. Thus, from the standpoint of amount of structural material required per unit area of mirror, it would appear desirable to decrease the size of the mirrors. However, if smaller mirrors were used, the cost per unit area for the guidance components would become excessive. Conversely if the cost per heliostat for the guidance components were to be altered significantly, it would be appropriate to re-evaluate both the optimum size and the cost for the mirror and pedestal components.
It will be seen that the features which make the multi heliostat central receiver collector the most promising candidate for large scale power plants also would in many cases be desirable for small and moderate sized collectors. However, it is clear that it would not be practical to control the individual mirrors of a small system with the same type of independent controls as have been proposed for the large scale system. Only the development of an appropriate system for mechanically linking the mirrors will make the multimirrored system with central receiver practical on a small scale.
Linked mirror systems providing concentration in two dimensions are known and may be found described, for example, in U.S. Pat. No. 3,861,379. However, two dimensional concentrators do not provide sufficiently high concentration ratios for many desired applications such as high temperature thermochemical energy storage, efficient conversion of solar energy to mechanical energy, or economical direct conversion of solar energy to electrical energy using a small area of photocell exposed to high flux. Additionally, collectors using two dimensional concentration tend to lose a significant amount of radiation out the end of the collector unless they are made quite long relative to their other dimensions.
U.S. Pat. No. 3,466,119 described a linked mirror collector providing three dimensional concentration, which is based on a special type of linkage incorporated into an equatorial mount. However, the basic linkage only allows collective control of the mirrors corresponding to changes in the hour angle of the sun. The patent describes an additional feature which is an intermittently operated mechanism designed to vary the orientation of the mirrors corresponding to changes in the declination of the sun. However, of the collectors of this type which have been constructed, apparently none have incorporated the declination adjustment feature. The declination of the mirrors on existing collectors of this type must currently be adjusted individually by hand on a periodic basis. It appears unlikely that use of the intermittent mechanism described in the patent would completely eliminate the need for periodic manual adjustment. The collectors of this type which have been constructed have been used as research instruments, so the requirement for frequent attention and adjustment has been acceptable. However, for routine use, the lack of declination control would not be tolerable.